Pressure Assisted Wafer Holding Apparatus and Control Method

ABSTRACT

An electrostatic wafer holding apparatus includes an electrostatic chucking pedestal and a bi-directional backside conduit in fluid communication with a backside of the chucking pedestal. The bi-directional backside conduit is in fluid communication with a backside carrier gas supply line, and is further in fluid communication with a vacuum supply line.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/710,653, filed Jul. 27, 2004, the disclosure of which is incorporatedby reference herein in its entirety.

BACKGROUND

The present invention relates generally to semiconductor deviceprocessing equipment, and, more particularly, to a pressure assisted,electrostatic wafer holding apparatus and control method.

Chucks are used in various material processing systems to retainworkpieces (e.g., semiconductor wafers, dielectric substrates, etc.)thereon in a mechanically stationary position while the system processthe workpiece. In particular, semiconductor wafer chucks are used tohold substrates in place and/or provide a heat transfer medium to thesubstrate for various processing steps (e.g., chemical vapor deposition,sputtering, etching, etc.) implemented during the manufacture ofsemiconductor devices such as integrated circuits.

There are three general types of chuck mechanism implementations thatmay be found in existing industrial wafer processing tools:gravity-based chucks, mechanical chucks and electrostatic chucks. In agravity based chuck, a wafer is free standing on the chuck and simplyheld in place by gravity. While this design is the simplest of thethree, a significant drawback thereof is the fact that there is anuncontrollable amount of contact between the wafer and the chuck. Thisis particularly disadvantageous for processes requiring heat transferbetween the wafer and the chuck, and especially in low-vacuum processingenvironments, as this results in a highly non-uniform temperaturegradient on the wafer. In turn, this leads to poor thickness control(e.g., for CVD/ALD processes), as well as variations in film properties.

In a mechanical chuck, a wafer is mechanically clamped to the chuck bymeans of a retention device, such as an edge ring. While this approachworks fairly well for wafers that are somewhat bowed, the use of amechanical retaining device can generate contaminating particles, aswell as create a shadowing effect around the edge of the wafer. In fact,mechanically based chucks are virtually non-existent for semiconductortooling of 300 mm wafer sizes.

On the other hand, an electrostatic chuck (ESC) retains a wafer thereonby generating a charge differential between a surface of the wafer andone or more electrodes located within the body of the chuck. The ensuingelectrostatic force developed between the wafer and the electrodesretains the wafer against the chuck body. The electrodes are typicallyinsulated from the wafer by a relatively thin layer of dielectricmaterial. There are several well-known techniques for generating theelectrostatic force in an electrostatic chuck. In addition, a typicalESC has an array of raised bumps, the surfaces of which are coated withcertain semi-conducting composites such that a high electric field maybe established, but not so as to excessively overload the power supply.For example, when a DC voltage ranging from about 200 volts to about 750volts is applied to the chuck, an electrostatic attraction between thewafer and the chuck is established. This will generally create goodcontact without the drawbacks associated with a mechanical chuck.

However, although electrostatic chucks are extensively used in many 200mm and 300 mm tool sets, there are still some disadvantages associatedwith electrostatic chucking. For example, electrostatic chucking forceis a function of temperature, as well as the backside material of thewafer. While a higher chucking force may be obtained at a higher chuckand wafer temperature, the raising of the wafer temperature may not be aviable option for certain processes if the raised temperature changesthe nature and/or rate of the particular process. Furthermore, forwafers that are more severely bowed (particularly, for a compressivelybowed wafer stack), there is insufficient initial contact between thebackside of the wafer and the chuck to take advantage of the generatedelectrostatic forces for adequate retention.

Accordingly, it would be desirable to be able to implement a chuckingapparatus and method that advantageously utilizes the advantages of bothelectrostatic and mechanical chucking, but that does not suffer from thedrawbacks associated therewith.

SUMMARY

The foregoing discussed drawbacks and deficiencies of the prior art areovercome or alleviated by an electrostatic wafer holding apparatus. Inan exemplary embodiment, the apparatus includes an electrostaticchucking pedestal and a bi-directional backside conduit in fluidcommunication with a backside of the chucking pedestal. Thebi-directional backside conduit is in fluid communication with abackside carrier gas supply line, and is further in fluid communicationwith a vacuum supply line.

In another embodiment, an electrostatic wafer holding apparatus includesan electrostatic chucking pedestal having an inner zone and an outerzone. A bi-directional backside conduit is in fluid communication with abackside of the chucking pedestal, and is in fluid communication with abackside carrier gas supply line. The bi-directional backside conduit isfurther in fluid communication with a vacuum supply line. The inner zoneand the outer zone of the chucking pedestal are mechanically decoupledfrom one another.

In still another embodiment, a method for implementing pressure assistedelectrostatic chucking includes placing a wafer onto an electrostaticchucking pedestal, and introducing a supply of backside carrier gas to abackside of the electrostatic chucking pedestal. The pressure betweenthe wafer and the electrostatic chucking pedestal is monitored todetermine whether a threshold level of chucking force exists. Thebackside carrier gas is decoupled from the backside of saidelectrostatic chucking pedestal, and the backside of the electrostaticchucking pedestal is coupled to a vacuum supply whenever the actuallevel of chucking force is less than the threshold level of chuckingforce.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numberedalike in the several Figures:

FIG. 1 is a cross-sectional view of an existing electrostatic chuck;

FIG. 2 is a graph that illustrates the effect of chucking force as afunction of temperature;

FIG. 3( a) is a schematic view of a chuck and wafer characterized by atensile film stack;

FIG. 3( b) is a schematic view of a chuck and wafer characterized by acompressive film stack;

FIG. 4( a) is a cross-sectional view of a pressure assisted,electrostatic wafer holding apparatus, in accordance with an embodimentof the invention;

FIG. 4( b) is a top view of the pressure assisted, electrostatic waferholding apparatus of FIG. 4( a), particularly illustrating theorientation of mini-dipoles on the raised bumps and gas channels of thechuck;

FIG. 5 is a process flow diagram of a method for implementing curvatureassisted chucking, in accordance with a further embodiment of theinvention;

FIG. 6 is a top view of a dual-zone electrostatic chuck, in accordancewith a further embodiment of the invention;

FIG. 7 is a cross-sectional view of the dual-zone electrostatic chuck ofFIG. 6;

FIG. 8( a) is a cross-sectional view of the dual-zone electrostaticchuck having a wafer characterized by a tensile film stack; and

FIG. 8( b) is a cross-sectional view of the dual-zone electrostaticchuck having a wafer characterized by a compressive film stack.

DETAILED DESCRIPTION

Referring initially to FIG. 1, there is shown a cross-sectional view ofan existing electrostatic chuck 100. As is known in the art, there aretwo main components of electrostatic chucking force. When a wafer 102 isfirst loaded onto the chuck 100, the gap size therebetween is relativelylarge and, as such, the main attractive force between the two isCoulombic in nature. Once the wafer 102 is attracted to the energizedchuck 100 and begins to be flattened as a result, the strongerJohnsen-Rahbek force then becomes the dominant attractive force.However, as indicated previously, conventional electrostatic chuckinghas certain inherent deficiencies associated therewith. For example,FIG. 2 is a graph that illustrates the effect of chucking force as afunction of temperature. Again, while a higher chucking force may beobtained at a higher chuck and wafer temperatures, this may not be aviable option for certain manufacturing processes.

Moreover, for severely bowed wafers (e.g., the concave wafer 302 ofFIGS. 3( a), the convex wafer 304 of FIG. 3( b)), there is not enoughcontact surface between the backside of the wafer and the chuck. This isparticularly the case for wafers having a negative radius of curvature(i.e., convex wafers having a compressively stressed film stack). Thus,the gap distance therebetween is out of the operating range of thestronger Johnsen-Rahbek force. This has become a well-known problem, forexample, in its inability to chuck certain SiGe wafers during aluminumstack deposition. Another specific difficulty arises in chucking 300 mmwafers for C4 plating liner seed deposition. For heavily bowed SiGewafers, it may even be necessary to move the wafer to another depositiontool having mechanical clamps. During the deposition of a metal stack,such as Ti/TiN/A1/Ti/TiN, for example, metals such as Ti ad A1 becomeoxidized in the presence of air. Thus, this is not a practical optionand thus the wafer is scrapped as a result.

Therefore, in accordance with an embodiment of the invention, there isdisclosed a pressure assisted, electrostatic wafer holding apparatus 400that utilizes a bi-directional backside conduit. As shown moreparticularly in FIGS. 4( a) and 4(b), the apparatus 400 includes achucking pedestal 402 with mini-dipoles 414 and gas channels 415. Abackside supply line 406 is used to provide a carrier gas to thebackside of the wafer through conduit 408 in fluid communicationtherewith, while a vacuum supply line 410 is also in selective fluidcommunication with conduit 406, and controlled by detection circuitry412. FIG. 4( b) is a top view of the pressure assisted, electrostaticwafer holding apparatus 400 of FIG. 4( a), particularly illustrating theorientation of mini-dipoles 414 on the raised bumps of the chuckingpedestal 402.

Under normal operation of apparatus 400, when a wafer applied thereto isrelatively flat, the conduit 408 is used to flow a small amount ofbackside gas for wafer detection and operation, as well as to improvethe thermal transport between the temperature controlled chuck and thewafer. On the other hand, if the film stack on the wafer is verycompressive, for example, the detection circuitry 412 detects the large(negative) curvature and switches the gas conduit 408 to a vacuum modeof operation. Simultaneously, a front side gas flowing into the chamber(not shown) increases the pressure differential such that the wafer willbe assisted in deflecting toward the chuck, thereby enabling theJohnsen-Rahbek force to take effect.

Referring now to FIG. 5, there is shown a process flow diagram of acontrol method 500 for implementing pressure assisted electrostaticchucking, in accordance with a further embodiment of the invention.Beginning at block 502, a wafer is positioned on an electrostatic chuck,such as apparatus 400 described above, for example. Initially, abackside gas is supplied to the chuck pedestal and the resultingpressure between the wafer and chuck is monitored, as shown in block504. If at decision block 506 it is determined that the pressure iswithin specified limits, then sufficient chucking force exists and thusnormal wafer processing may proceed as shown at block 508.

However, if sufficient chucking force is not detected, then method 500proceeds to block 510 where front side gas flow is introduced, and thegas conduit is coupled to the vacuum supply in an attempt to achievesufficient chucking force. Then, decision block 512 again determineswhether the pressure is now within specified limits. If this is thecase, then the vacuum supply is removed, the backside gas supply isrestored and normal wafer processing may proceed. However, if there isstill insufficient chucking force, then method 500 proceeds to decisionblock 514 to see whether the electrostatic chuck voltage is less thanthe maximum allowed value. If the electrostatic chuck (ESC) voltage doesnot exceed the maximum value, then the ESC voltage is increased at block516 before method 500 returns to block 512. This sequence may continueuntil the chucking force becomes sufficient or until the ESC voltageexceeds the maximum value. As further shown at decision block 514, whenthe ESC voltage exceeds the maximum value (and the pressure is still notwithin the specified limits), the process exits at 518, and the wafer isdeemed to be too bowed (or broken), and will be scrapped as defective.

Referring generally now to FIGS. 6 and 7, there is shown a dual-zoneelectrostatic chuck 600, in accordance with an alternative embodiment ofthe invention, in which an inner zone 602 and an outer zone 604 aremechanically decoupled from one another. In particular, the height ofthe inner zone 602 is independently adjustable with respect to theheight of the outer zone 604, and vice versa. As illustrated in FIG. 7,adjustment of the height of the zones may be accomplished through asuitable micro-positioning control mechanism, such as through steppingmotor 606 and extendable shaft 608 assemblies mounted on a base 610. Asis the case with the embodiment of FIGS. 4( a) and 4(b), the dual-zoneESC 600 is provided with both a carrier gas supply 612 and a vacuumsupply 614.

The advantages of the dual-zone embodiment are appreciated uponconsideration of the condition in which a wafer is curved either in aconcave or a convex orientation. In the case where the wafer 616 has arelatively large radius of curvature, the micro-positioning control canraise the outer zone 604 slightly with respect to the inner zone 602 soas to bring the inner zone 602 in closer proximity to the center portionof the wafer 616, as shown in FIG. 8( a). Conversely, if the wafer 616has a relatively large negative radius of curvature, then the outer zone604 may be retracted slightly with respect to the inner zone 602 so asto bring the outer zone 604 in closer proximity to the outer portions ofthe wafer 616, as shown in FIG. 8( b). Once the bowed wafer is flatteneddue to the assisted electrostatic forces, the chucking voltages in bothzones may be reduced before continuing with normal processing. In eitherinstance, it will be seen that the closer physical proximity of thewafer and chuck pedestal using the dual-zone embodiment will furtherfacilitate a stronger chucking force and improve device yield.

While the invention has been described with reference to a preferredembodiment or embodiments, it will be understood by those skilled in theart that various changes may be made and equivalents may be substitutedfor elements thereof without departing from the scope of the invention.In addition, many modifications may be made to adapt a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to the particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe appended claims.

1. An electrostatic wafer holding apparatus, comprising: anelectrostatic chucking pedestal configured for wafer retentionthereupon, said electrostatic chucking pedestal having a plurality ofgas channels formed through a top surface thereof; a bi-directionalbackside conduit in fluid communication with a backside of said chuckingpedestal and said plurality of gas channels; said bi-directionalbackside conduit in fluid communication with a backside carrier gassupply line; said bi-directional backside conduit further in fluidcommunication with a vacuum supply line, wherein said plurality of gaschannels are configured to facilitate vacuum assisted chucking of awafer retained on said electrostatic chucking pedestal, during whichvacuum assisted chucking an electrostatic chucking voltage remainsapplied to the electrostatic chucking pedestal, wherein the vacuumassisted chucking is implemented prior to performing a wafer processingoperation for which the wafer is chucked; a mechanism for selectivelycoupling said bi-directional backside conduit to one of said backsidecarrier gas supply line and said vacuum supply line; and detectioncircuitry for detecting a curvature present in a wafer placed on saidchucking pedestal, said detection circuitry configured to cause saidbi-directional backside conduit to be decoupled from said backsidecarrier gas supply line and coupled to said vacuum supply line upon saiddetecting a curvature present in said wafer, and said detectioncircuitry further configured to cause said bi-directional backsideconduit to be decoupled from said vacuum supply line and re-coupled tosaid backside carrier gas supply line upon detecting a desired pressurebetween said wafer and said chucking pedestal.
 2. An electrostatic waferholding apparatus, comprising: an electrostatic chucking pedestalconfigured for wafer retention thereupon, said chucking pedestal havingan inner zone and an outer zone, the inner and outer zones each having atop surface disposed beneath a wafer placed on said chucking pedestal; abi-directional backside conduit in fluid communication with a backsideof said chucking pedestal; said bi-directional backside conduit in fluidcommunication with a backside carrier gas supply line; saidbi-directional backside conduit further in fluid communication with avacuum supply line; wherein said inner zone and said outer zone aremechanically decoupled from one another such that the top surface of theouter zone is capable of selective adjustment to positions both belowand above the top surface of the inner zone; and wherein saidelectrostatic chucking pedestal further comprises a plurality of gaschannels formed through a top surface thereof, said plurality of gaschannels also in fluid communication with said bi-directional backsideconduit, and wherein said plurality of gas channels are configured tofacilitate vacuum assisted chucking of a wafer retained on saidelectrostatic chucking pedestal, during which vacuum assisted chuckingan electrostatic chucking voltage remains applied to the electrostaticchucking pedestal, and wherein the vacuum assisted chucking isimplemented prior to performing a wafer processing operation for whichthe wafer is chucked; a mechanism for selectively coupling saidbi-directional backside conduit to one of said backside carrier gassupply line and said vacuum supply line; and detection circuitry fordetecting a curvature present in a wafer placed on said chuckingpedestal, wherein said detection circuitry is configured to cause saidbi-directional backside conduit to be decoupled from said backsidecarrier gas supply line and coupled to said vacuum supply line upon saiddetecting a curvature present in said wafer, and wherein said detectioncircuitry is further configured to cause said bi-directional backsideconduit to be decoupled from said vacuum supply line and re-coupled tosaid backside carrier gas supply line upon detecting a desired pressurebetween said wafer and said chucking pedestal.
 3. The apparatus of claim2, further comprising a suitable micro-positioning control mechanismassociated with each of said inner and outer zones of said chuckingpedestal, wherein a height of said inner and outer zones areindependently adjustable with respect to one another.
 4. The apparatusof claim 3, wherein said outer zone is configured to be in a raisedposition with respect to said inner zone when a wafer having a positiveradius of curvature with respect to said chucking pedestal is placedupon said chucking pedestal.
 5. The apparatus of claim 4, wherein saidinner zone is configured to be in a raised position with respect to saidouter zone when a wafer having a negative radius of curvature withrespect to said chucking pedestal is placed upon said chucking pedestal.6. The apparatus of claim 2, wherein said inner zone is concentricallydisposed with respect to said outer zone.